linux-kernel - Re: [PATCH 06/14] md/bcache: Use %pS printk format for direct addresses

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Message-ID: <20170907080519.GB533@jagdpanzerIV.localdomain>
Date:   Thu, 7 Sep 2017 17:05:19 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Helge Deller <deller@....de>
Cc:     Coly Li <i@...y.li>, linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Petr Mladek <pmladek@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-bcache@...r.kernel.org, linux-raid@...r.kernel.org
Subject: Re: [PATCH 06/14] md/bcache: Use %pS printk format for direct
 addresses

On (09/07/17 09:42), Helge Deller wrote:
> >> -		seq_printf(f, "%p: %pF -> %pf p %p r %i ",
> >> +		seq_printf(f, "%p: %pS -> %pf p %p r %i ",
> >>  			   cl, (void *) cl->ip, cl->fn, cl->parent,
> >>  			   r & CLOSURE_REMAINING_MASK);
> >>  
> >> @@ -187,7 +187,7 @@ static int debug_seq_show(struct seq_file *f, void *data)
> >>  			   r & CLOSURE_SLEEPING	? "Sl" : "");
> >>  
> >>  		if (r & CLOSURE_WAITING)
> >> -			seq_printf(f, " W %pF\n",
> >> +			seq_printf(f, " W %pS\n",
> >>  				   (void *) cl->waiting_on);
> >>  
> >>  		seq_printf(f, "\n");
> >>
> > 
> > It is unclear to me, that if %pF is used, on ia64/ppc64/parisc64 a
> > function descriptor conversion happens, what negative effect exactly
> > takes place ?
> 
> On ia64/ppc64/parisc64 the kernel will crash here in the worst case, because
> vsprintf() will try to read a pointer from that address and resolve it. 

probe_kernel_address() handles the page fault and returns -EFAULT if
you give it bad pointer. module_address_lookup() and get_symbol_pos()
seems to be smart enough not to crash on bad pointer as well. what am
I missing? could you please explain where we will crash?

	-ss